Many attempts have been conventionally proposed to manufacture a superconducting wire rod by depositing a film of oxide superconductor on a substrate.
Among them, a superconducting wire rod obtained by depositing an oxide superconductor represented by a composition formula of REBa2Cu3O7−δ (RE represents a rare-earth element; also referred to as 123-based or RE-based superconductor) on a tape-shaped metal substrate to have flexibility exhibits high degrees of current properties, and thus it is now one of superconducting wire rods on which research and development has been actively carried out. A number of prototypes for an electric power device or the like using such wire rods have already been manufactured.
The oxide superconductor has electrical anisotropy such that the flow of electricity is promoted along the crystal axes a and b of the crystals itself, but the flow of electricity is impended along the crystal axis c of the crystals itself. Accordingly, when the oxide superconductor is formed on a substrate, it is necessary to orient the crystal axes a and b so as to promote the flow of electricity and orient the crystal axis c along another direction.
However, the substrate itself is an amorphous material or polycrystalline material that has a crystal structure significantly different from that of the oxide superconductor. Therefore, it is difficult to form an oxide superconductor exhibiting high degrees of crystalline orientation on the substrate. Differences in coefficients of thermal expansion and lattice constants between the substrate and the oxide superconductor may cause distortion in the superconductor or peeling-off of the oxide superconductor film from the substrate during a cooling process to a superconducting critical temperature.
As an approach to address these problems, an orientation layer (i.e. intermediate layer) is first provided on a metal substrate, and an oxide superconductor is formed on the oriented layer. The oriented layer is deposited, for example, by an ion beam assisted deposition method (IBAD method) using a material such as MgO which exhibits high degrees of orientation of the crystal axis c and high degrees of in-plane orientation of the crystal axis a (i.e. biaxial crystal orientation).
Patent Literature 1 discloses a technique to achieve a higher degree of biaxial crystal orientation, in which a cap layer made of CeO2, PrO2 or the like is formed on an oriented layer, and then an oxide superconducting layer made of an RE-based superconductor containing Ba is formed thereon.
Patent Literature 2 discloses a technique for forming a high quality oxide superconducting thin film of epitaxial single crystal on a substrate having a single crystal structure of K2MnF4. However, when, as in a superconducting wire rod, the superconducting material is relatively large in length (including an elongated wire) and needs to be flexible, it is difficult to use a single crystal for a substrate.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2010-103021    [Patent Literature 2] JP-A No. 1991-069596